A liquid chromatograph is composed of a plurality of units, such as an auto-sampler, a pump and a column oven. Operations of those units are controlled according to control signals from a control system.
In recent years, a control system consisting of a personal computer in which a predetermined controlling and processing program is installed is widely used also for such liquid chromatographs in order to generally control analyzing units of the device and to process the thereby collected data. Such a control system has the function of preparing a schedule table in advance of an analysis and automatically performing a continuous analysis of a plurality of samples or similar analyses according to the prepared schedule (for example, see Patent Document 1).
FIG. 7 is one example of the schedule table for a liquid chromatographic analysis. In this table, each row corresponds to one analysis and consists of fields in which necessary information for that analysis is written, such as the sample name, the amount of sample injection, the name of a method file, and the name of a data file in which an analysis result is to be stored. A method file is a file in which operating conditions of the units constituting the liquid chromatograph are specified (those conditions are hereinafter called the “analysis method”). The information written in the file includes various parameters, such as the kinds of mobile phase and column to be used in the analysis as well as the flow rate of the pump and the temperature of the oven during the analysis.
After the schedule table is thus prepared, when the initiation of the analysis is ordered, an analysis of a number of samples is automatically performed, with the samples sequentially selected and the analysis conditions appropriately set according to the schedule.
In such a liquid chromatograph, one sample may be subjected to an analysis under various conditions so as to find an optimal condition for analyzing that sample. Such a technique is called the “method scouting.” In the method scouting, the user previously prepares a plurality of different method files with various combinations of the aforementioned parameters, and furthermore, as shown in FIG. 7, assigns a different method file to each row of the schedule table, with the same sample name and the same amount of sample injection specified in all the rows, before ordering the initiation of the analysis. As a result, a series of analyses are sequentially performed under various conditions as described in the method files specified in each row of the table. The chromatogram data obtained as the analysis result are stored, for each analysis, into one data file and saved in a storage device, such as a hard disk drive. The user refers to the chromatogram data stored in the storage device and find the analysis condition under which an optimal analysis result has been obtained. The analysis condition thus found is chosen as the analysis method to be applied in an analysis of the sample in question.
As one of the analytical techniques using a liquid chromatograph, a gradient liquid-sending method is known. In this method, a mixture of two or more solvents having different natures, such as water and an organic solvent, is used as a mobile-phase liquid to be sent into a column, with the mixture ratio of the solvents being varied with time. This technique is particularly useful for clearly separating a multi-component sample into its components.
To perform an analysis of a sample by the gradient liquid-sending method (which is hereinafter called the “gradient analysis”), a user sets a gradient profile (e.g. as shown in FIG. 6) as one of the analytical parameters to be included in the method file. The gradient profile shows the target values of the mobile-phase composition with respect to the lapse time from the beginning of the analysis. The example of FIG. 6 is a profile for a gradient analysis in which a mixture of solvents A and B is used as the mobile phase. The composition of the mobile phase is represented by the percentage of the solvent B in the mixture. A solvent with a low eluting power (e.g. a more polar solvent in the case of a reverse-phase analysis) is used as the solvent A, while a solvent with a high eluting power (e.g. a less polar solvent in the case of a reverse-phase analysis) is used as the solvent B. After a sample is injected at time t0, the percentage of the solvent B is initially maintained at a low level for a predetermined period of time (from t0 to t1), whereby the components in the sample are temporarily adsorbed in the column. After that, the percentage of the solvent B is linearly increased with time (from t1 to t2), whereby the components are sequentially and individually eluted from the column according to their properties (e.g. the polarity). Subsequently, the percentage of the solvent B is maintained at a high level for a certain period of time (from t2 to t3) so as to discharge residual components from the column, after which the mobile phase is restored to its initial composition. This state is further maintained for a certain period of time (from t3 to t4) until the inside of the column is equilibrated.
In the following description, the process corresponding to the period of time from t0 to t1 is called the “sample injection process”, the process corresponding to the period of time from t1 to t2 is called the “gradient process”, and the process corresponding to the period of time from t2 to t3 is called the “equilibrating process”. In some cases, the sample injection process is omitted and the gradient process is initiated simultaneously with the injection of the sample.